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Setting Up a Lexical Analyser and Parser in Ruby

I wrote this post as I was setting up the lexer and parser for Rubex, a new superset of Ruby that I’m developing.

Let’s demonstrate the basic working of a lexical analyser and parser in action with a demonstration of a very simple addition program. Before you start, please make sure rake, oedipus_lex and racc are installed on your computer.

Configuring the lexical analyser

The most fundamental need of any parser is that it needs string tokens to work with, which we will provide by way of lexical analysis by using the oedipus_lex gem (the logical successor of rexical). Go ahead and create a file lexer.rex with the following code:

In the above code, we have defined the lexical analyser using Oedipus Lex’s syntax inside the AddLexer class. Let’s go over each element of the lexer one by one:

macro

The macro keyword lets you define macros for certain regular expressions
that you might need to write repeatedly. In the above lexer, the macro DIGIT is a regular expression (\d+) for detecting one or more integers. We place the regular expression inside forward slashes (/../) because oedipus_lex requires it that way. The lexer can handle any valid Ruby regular expression. See the Ruby docs for details on Ruby regexps.

rule

The section under the rule keyword defines your rules for the lexical analysis. Now it so happens that we’ve defined a macro for detecting digits, and in order to use that macro in the rules, it must be inside a Ruby string interpolation (#{..}). The line to the right of the /#{DIGIT}/ states the action that must be taken if such a regular expression is encountered. Thus the lexer will return a Ruby Array that contains the first element as :DIGIT. The second element uses the text variable. This is a reserved variable in lex that holds the text that the lexer has matched. Similar the second rule will match any character (.) or a newline (/n) and return an Array with [text, text] inside it.

inner

Under the inner keyword you can specify any code that you want to occur inside your lexer class. This can be any logic that you want your lexer to execute. The Ruby code under the inner section is copied as-is into the final lexer class. In the above example, we’ve written an empty method called do_parse inside this section. This method is mandatory if you want your lexer to sucessfully execute. We’ll be coupling the lexer with racc shortly, so unless you want to write your own parsing logic, you should leave this method empty.

Configuring the parser

In order for our addition program to be successful, it needs to know what to do with the tokens that are generated by the lexer. For this purpose, we need racc, an LALR(1) parser generator for Ruby. It is similar to yacc or bison and let’s you specify grammars easily.

Go ahead and create a file called parser.racc in the same folder as the previous lexer.rex and Rakefile, and put the following code inside it:

As you can see, we’ve put the logic for the parser inside the AddParser class. Yacc’s $$ is the result; $0, $1… is an array called val, and $-1, $-2… is an array called _values. Notice that in racc, only the parsing logic exists inside the class and everything else (i.e under header and inner) exists outside the class. Let’s go over each part of the parser one by one:

class AddParser

This is the core class that contains the parsing logic for the addition parser. Similar to oedipus_lex, it contains a rule section that specifies the grammar. The parser expects tokens in the form of [:TOKEN_NAME, matched_text]. The :TOKEN_NAME must be a symbol. This token name is matched to literal characters in the grammar (DIGIT in the above case). token and expr are varibles. Have a look at this introduction to LALR(1) grammars for further information.

header

The header keyword tells racc what code should be put at the top of the parser that it generates. You usually put your require statements here. In this case, we load the lexer class so that the parser can use it for accessing the tokens generated by the lexer. Notice that header has 4 hyphens (-) and a space before it. This is mandatory if your program is to not malfunction.

inner

The inner keyword tells racc what should be put inside the generated parser class. As you can see there are two methods in the above example - next_token and prepare_parser. The next_token method is mandatory for the parser to function and you must include it in your code. It should contain logic that will return the next token for the parser to consider. Moving on the prepare_parser method, it takes a file name that is to be parsed as an argument (how we pass that argument in will be seen later), and initialzes the lexer. It then calls the parse_file method, which is present in the lexer class by default.

The next_token method in turn uses the @lexer object’s next_token method to get a token generated by the lexer so that it can be used by the parser.

Putting it all together

Our lexical analyser and parser are now coupled to work with each other, and we now use them in a Ruby program to parse a file. Create a new file called adder.rb and put the following code in it:

The prepare_parser is the same one that was defined in the inner section of the parser.racc above. The do_parse method called on the parser will signal the parser to start doing it’s job.

In a separate file called text.txt put the following text:

2+2

Oedipus Lex does not have a command line tool like rexical for generating a lexer from the logic specified, but rather has a bunch of rake tasks defined for doing this job.
So now create a Rakefile in the same folder and put this code inside it:

Running rake parser will generate a two new files - lexer.rex.rb and parser.racc.rb - which will house the classes and logic for the lexer and parser, respectively. You can use your newly written lexer + parser with a ruby adder.rb text.txt command. It should output 4 as the answer.